LBHA is a system in which a hydraulic actuator driven by an airframe hydraulic system of an airplane is supplemented by an emergency hydraulic power source unit (Local Pump Unit abbreviated as LPU). Because of this LPU, the function of the hydraulic actuator is maintained even after the airframe hydraulic system loses its function. LBHA is recited in, for example, the specification of German Laid-Open Patent Application No. 19654781 (hereinafter, Patent Document 1). The LBHA recited in Patent Document 1 is arranged so that two relief valves (9, 10) and a control valve (8) are disposed on a return path of pressure fluid. For example, the relief valve (9) is arranged to be a low-pressure relief valve whereas the relief valve (10) is arranged to be a high-pressure relief valve.
The relief valve (9) functions as a low-pressure relief valve when the airframe hydraulic system is in normal operation. In the meanwhile, when the airframe hydraulic system loses its function, the control valve (8) cuts off the relief valve (9) and the hydraulic pump (5) is electrically driven. In so doing, the relief valve (10) prevents the hydraulic fluid in the closed loop from leaking out (shut off function) and prevents the pressure inside the closed loop from rapidly increasing on account of the thermal expansion of the hydraulic fluid (thermal relief function). The shut off function and the thermal relief function are collectively termed as high pressure relief functions.
As such, the LBHA system of the Patent Document 1 is arranged to exert plural functions by two relief valves (9, 10) and a control valve (8).
This LBHA system of Patent Document 1 is disadvantageous in that the LPU and LPHA require a large space inside the airplane because two relief valves (9, 10) and a control valve (8) must be provided.